Hades on Ruby is a very very low grade hit, like less than 200hp. But the summon will cast a Pentragram with a Grim Reaper toiling over a bubbling couldron, where it will cast its flame and petrifying curse. The effect on Ruby, despite its extremely light hit, is enormous in that the affect lasts up to two KOTR from one person, Cloud.
Now...imagine, it took me 6 casts of KOTRs from Cloud (and Hades for every 2 KOTRs)...before I decided to awaken my two companions from their "deaths". This time, I had the two Mime Cloud\'s summons. That\'s 6 KOTRs before Ruby broke its curse...only to be petrified again. Its cheating...but that long battle with Emi had me a bit skittish for a fair fight.
Also, I had MP Absorb connected to my Master Summon (or mastered KOTR) so that I never run out. The absorption for each KOTR is close to 1000 magic points. Basically, you never run out of magic.